Disguised Blessings: A Mechanistic Understanding of the Beneficial Outcomes Triggered by Partial K Replacement With Na in Two Eucalyptus Species Under Drought Stress.

While not essential for most plants, sodium (Na) can partially substitute for potassium (K) in some metabolic functions. Thus, understanding the mechanisms underlying K and Na uptake, transport, utilization, and ion replacement is crucial to sustain forest production. A pot experiment was designed with 6 K/Na ratios (100/0, 85/15, 70/30, 55/45, 40/60, and 0/0%) and two water conditions (well-watered, W+; and water-stressed, W-) on two Eucalyptus species with contrasting drought tolerance. In a multi-level analysis, we measured morphological, nutritional, physiological, biochemical, molecular, and anatomical traits. Low to moderate K replacement with Na (85/15%-55/45%) provided partial and faster stomatal closure (lower δC), thereby enhancing plants' water status (WUE, RLWC, Ψ, Ψ), photosynthetic capacity (g, E, A, C/C), photoprotection (NPQ, qP, ETR, F/F, ΦPSII), and growth (height, collar diameter, LA, TDM) relative to exclusive K supply. The 70/30% K/Na replacement was defined as the ideal ratio, upregulating K and water uptake (overexpression of AKT1, PIP2;5, PIP2;7 and TIP1;3), maximizing enzymatic antioxidant performance and biomass production, and reducing oxidative stress. High K replacement with Na (40/60%) and K deficiency (0/0%) led to incomplete stomatal closure reduced water status, photosynthetic capacity, photoprotection, and growth, especially in the species with low drought tolerance.